Line Ib lymphocytic Leukemia in C58 strain mice will be used as a model system in which to study the role of individual subpopulations of T-lymphocytes in the protective immune response against leukemia. Antisera to Ly antigen and to Ia antigen markers will be used both as a primary means of separating populations and together with other methods as a probe to assess biophysical and pharmacological methods of cell separation. Using various effector cell-target cell combinations, the cytotoxic and cytostatic reactivity of purified lymphocytes on radioisotope labelled Ib leukemia cells wil be determined. The influence of soluble antigens and of interaction between lymphocyte subpopulations will be assayed at the effector cell and target cell levels. In vitro reactions will be correlated with protection in vivo by quantitative adoptive transfer assays. Ly markers will be monitored in vivo in an effort to show how changes in T-cell subpopulations are related to susceptibility and resistance to leukemia.